1. Field of the Invention
The present invention relates, in general, to a system for electrochemically deburring perforate metallic clad dielectric laminates such, for example, as printed circuit boards (PCBs) or printed wiring boards (PWBs) and the like; and, more particularly, to methods and apparatus wherein the metallic clad dielectric laminate to be deburred is immersed within a suitable conventional electrolyte solution, one external metallic conductive layer or clad on the laminate is coupled to the positive terminal of a suitable power supply so that the clad comprises the anode in an anodic deburring process, and one or more conductive paths are established through the electrolyte solution passing through the thru-hole perforation(s) in the dielectric laminate between the anode and a suitable cathode, thereby concentrating current density at those portions of the anodic clad in the regions of the peripheral edges of the thru-hole perforations in the dielectric laminate and preferentially removing burrs, protrusions and irregularities therefrom to form a smooth rounded edge in the anodic metal clad surrounding each hole.
Those skilled in the art have long recognized that in conventional electroplating operations, edges, protrusions, sharp angles and the like will be preferentially plated--generally an undesirable phenomenon, particularly when dealing with printed circuit boards or printed wiring boards and the like--and, this fact has resulted in many sophisticated techniques and different types of apparatus for minimizing the preferential plating problem. On the other hand, electrochemical deburring constitutes precisely the opposite process--i.e., deplating as contrasted with plating--and takes advantage of such negative phenomenon. That is to say, in a deburring operation, the desired end result is the removal of the burrs or other protrusions while simultaneously minimizing deplating of relatively smooth plated areas; and, consequently it is highly desirable that the drilling burrs and similar protrusions function as sites for preferential electrochemical attack. At the same time, however, it is also desirable that the deburring system be such that only minimal attack occurs at the relatively smooth portions of the clad dielectric laminate being worked upon.
2. Prior Art
In general, printed circuit boards or printed wiring boards and the like commonly comprise plated or clad dielectric laminates having at least one, and normally two, external relatively thin spaced layers or clads of copper--e.g., clads on the order of from about 1.3 mils thick to as thin as perhaps 0.16 mil--with an intermediate non-conductive dielectric layer commonly formed of a fiberglass/epoxy material which is generally on the order of about 59 mils in thickness. Such workpieces are commonly drilled to form a multiplicity of thru-hole perforations in each metal clad dielectric laminate for mounting of conventional electronic components and/or accommodating electrical leads or the like. Unfortunately, such drilling and similar machining operations commonly cause the formation of burrs and similar irregularities in the region of the hole edges, which burrs are highly undesirable.
Prior to the advent of the present invention, the most commonly employed expedient for deburring printed circuit boards or printed wiring boards and the like involved a mechanical abrasive operation such, for example, as sanding (both manually and automatically), spinning employing abrasive roller-type machines (both oscillating and non-oscillating types), and pressure blast equipment wherein an abrasive is sprayed under high pressure against the board to remove the burrs. The particular deburring equipment employed will generally vary as a function of the normal burr height encountered--a dimension that can range from a few ten-thousandths of an inch to several thousandths of an inch depending on the condition of the drill and drilling parameters. The most popular choice has heretofore been the oscillating, spinning, abrasive roller-type machine. These machines perform an adequate job of deburring while radiusing the hole edges to a limited degree. Often, however, burrs are folded over into the hole and, if not removed, such folded over burrs can cause significant problems during subsequent processing operations. For example one of the principal problems is the reduction of the finished hole size due to nodule growth at the site of the folded over burr. When plating, other recognized problems associated with burrs, especially burrs which have been turned over, include entrapment of processing solution and/or air bubbles. Either of these conditions can cause significant weakening of the thru-hole copper plating at its most vulnerable point--viz., the corner of the clad surrounding the peripheral edge of the thru-hole perforation.
It is further well-known to those skilled in the art that the effectiveness of mechanical deburring operations is also fairly operator-dependent. That is, the operator is relied upon to make adjustments for varying board thicknesses, to stagger parts through conveyerized feeds, and to dress the wheels as they become out of balance or wear unevenly. Moreover, such equipment is capital-intensive and disadvantageous in that it is commonly limited as to the dimensions of the part that can be accepted and processed. Thus, such systems are commonly constrained to operate with piece parts having a 24" maximum width, a 6" to 8" minimum width, and a minimum thickness of about 0.010".
Another known conventional approach for deburring perforate metallic clad dielectric laminates involves a purely chemical deburring process such, for example, as a process described by H. V. Connely and B. E. Rothschild of the Autonetics Division of Rockwell International; a process employing a sulfuric-peroxide solution to chemically remove burrs.
Electrochemical deburring, etching and/or polishing of solid piece parts have, of course, long been recognized in the art. One disclosure of interest is that found in Brandt et al U.S. Pat. No. 2,590,927 wherein the patentees disclose an alternating current process for deburring the metal portion of metal/insulation laminations in magnetic core ends. Similar disclosures are found in Smith U.S. Pat. No. 2,559,263 (an electropolishing process); Armstrong et al U.S. Pat. No. 2,850,444, and Amaya U.S. Pat. No. 2,890,159 (etching of semi-conductor surfaces); McCord et al U.S. Pat. No. 2,904,479 (a rare metal polishing process utilizing a near-neutral ammonium fluoride solution); Scott U.S. Pat. No. 2,963,411 (a process for exposing and etching defects comprising shorts in p-n-type junctions); Lane U.S. Pat. No. 3,027,310 (a metal cleaning operation involving separate baths); and, Lanning et al U.S. Pat. No. 3,468,775 (an electrochemical machining process for removing burrs from a conductive workpiece).
One problem inherent with electrochemical deplating processes has been the undesired channeling effect that occurs in the metallic portions of the workpiece as a result of off-gasing and/or the movement of air bubbles over the surface of the workpiece during a deplating operation.
Yet, notwithstanding the extensive development of the electroplating and electrochemical polishing, etching, deburring and/or other deplating arts, virtually the only commercially employed process for removing burrs from perforate metallic clad dielectric laminates such as PCBs or PWBs has been the foregoing mechanical abrading processes hereinabove described--this despite the known disadvantages of such systems.